It is estimated that as many as 3,000 alien species per day are transported throughout the world in the holds of ships. Most of these organisms do not survive the stress of the voyage and are not capable of competing in their new environments when they are discharged in a remote port. However, the records are replete with thousands of examples where alien species have thrived in their new environments and gone on to wreak ecological and economic havoc in their new surroundings. The most celebrated example of recent times is the zebra mussel (Dreisenna polymorpha) infestation of the Great Lakes and the Mississippi and Hudson Rivers. The infestation was established in the middle of the last decade and their numbers have exploded because of their efficient reproduction. The zebra mussels cling tenaciously to surfaces such as water and drain pipes and soon cause flow problems in water treatment plants and cooling towers. The infestations are expected to cost $5 billion in control efforts. Another rapidly spreading exotic specie in the Great Lakes ecosystem is the Eurasian ruffe, a foraging fish related to North American perch. Native fish populations are seriously affected by the feeding and reproduction capacity of the ruffe. Countless examples of alien or exotic species becoming established exist in other areas of the world, as well.
Regulation of ballast water management and treatment will be required to lessen the probability of alien species introduction and the concomitant economic and environmental impacts. The currently favored approach to mitigating the threat from ballast water is to exchange the water in the open ocean during transit. This practice can lower the density of larger organisms in the water from 170 to 2 per metric ton. Several shortcomings of the procedure exist, however. Safety of the ship is compromised when ballast is exchanged in open waters. Care must be exercised to avoid adversely affecting the performance of the ship in high seas and to avoid causing mechanical failure due to unsymmetrical loading of the ship. Thus, some ships are hesitant to adopt the practice. Even ships willing to comply with the practice can only manage to do so about 40% of the time. Also, a fraction of the ballast water is not pumpable and remains in the compartment along with sediments. Organisms may establish permanent or semi-permanent communities in this environment and serve as a reservoir of infestation to freshly loaded ballast. It is recognized that ballast exchange is neither practical nor effective as a means of treating ballast water, and that either land- or water-based treatment facilities are needed to effectively deal with the problem.
Most ships have a ballast water capacity that is 25 to 30 percent of their dead weight tonnage, with 25 percent being the norm. The volume of total ballast can range from 13,500 to 9.3 million gallons. An average ballasting rate is on the order of 300,000 gallons per hour, while an average deballasting rate is around 1 million gallons per hour for as long as 10 hours. Pumping and treating capacity of up to 10 million gallons per ship deballasting may ultimately be required of a treatment facility located dockside at the port. Dockside facilities would require space for constructing a treatment plant that could handle flows of that magnitude and the infrastructure and capacity to service all ships unloading in port.
On board treatment systems are increasingly considered to be a viable alternative. By taking on clean water, the transport of organisms is eliminated. Filtration of incoming ballast at the 251 μm size range has been determined to be an effective method of reducing the loading of zooplankton and phytoplankton in incoming ballast. Backflush water is simply discharged back into the harbor of origin. Secondary treatment options, such as ozone or UV, could be coupled with filtration for deactivation of smaller organisms. Treatment could also be carried out while the ship was under way, but the transit time may not be adequate. Shipboard treatment is hindered by the lack of space for installing the appropriate equipment, and sometimes by the lack of a suitable power source for pumping ballast water for treating. Treatment during deballasting may require a larger capacity system, and substances from the hold or ballast tanks, such as iron or sediment, can interfere with the treatment.
Ballast water presents unique challenges to conventional disinfection technologies owing to the large number of organisms, the diversity of their composition, and the chemical and physical characteristics of ballast water. Some characteristics of conventional technologies that have been applied to ballast water treatment are outlined, below.
Filtration:
Microfiltration removes most unwanted organisms; though, it may be too costly for large ships. The backflushing rate is excessive at 25 μm. 50 μm particle size removal is achievable, but 100 μm may be the practical limit. Microorganisms and dinoflagellates pass. Most likely, filtration is used in combination with UV or chemical biocides.
Thermal:
The water is heated to 60° C. for pasturization, which requires about 90 MW of energy on average ship. However, often only 20 MW is available. Moreover, thermal treatment generally is not practical in northern climates.
UV Irradiation:
Ultraviolet (UV) irradiation can be effective if water is not turbid. Only a short contact time is required, and the equipment can be had for a small capital expenditure. A portable unit may be immersed in ballast tanks for treatment during transit. There are no hazardous reagents used nor byproducts produced. UV irradiation requires filtration or cyclone pre-treatment to maintain low turbidity. UV irradiation is not effective against higher organisms or cysts.
Chlorine:
Chlorine is effective at high doses and long contact times. However, chlorine is corrosive and hazardous to store and handle. Chlorine also produces chlorinated hydrocarbon byproducts. Though chlorine treatment is relatively inexpensive, the gas is potentially lethal.
Ozone:
Ozone can be effective at low doses and short contact times; ozone demand increases the necessary dosage. In previous methods, ozone was applied within the ballast tanks, where the effectiveness of ozone can be reduced by organic matter from sediment, and the effectiveness of the ozone was compromised in that ozone does not readily penetrate/diffuse through the sediment in the ballast, which is where many organisms reside. Bromine reactions and formation of halohydrocarbons were also a major concern. Further still, ozone treatment systems tend to be expensive, with a large initial capital investment. Ozone gas is also toxic; accordingly, monitoring equipment is employed while the system is operating. Ozone gas also has a corrosive effect on the ballast tank when employed in the tank. Nevertheless, ozone kills even highly resistant forms such as spores and cysts. Moreover, no hazardous reagent storage is needed, as ozone can be generated from dry air on demand.
Ozone is a mature technology, having been in use for over a century as a water and waste water treatment technology. The contact dosage needed to kill invertebrates in typical applications is on the order of 0.3 mg/L; and the most-resistant organisms, such as Cryptosporidium oocysts, can be killed via exposure to an ozone concentration of about 1.5 mg/L for 1 minute. Ozonation can be used to disinfect microorganisms, oxidize Fe2+ and Mn2+, control taste and odor, enhance coagulation-flocculation and remove color. An allotrope of oxygen, ozone is a highly reactive gas with a pungent odor having a standard oxidation-reduction potential of 2.08 volts. Because of this reactivity, the chemistry of the water will have an effect on the amount of ozone required for inactivation of organisms. Ozone readily attacks natural organic matter present in the water. At high pH (e.g., about 12), ozone may decompose to form the extremely reactive hydroxyl radical, which readily reacts with the carbonate or bicarbonate in waters. In ocean brines, ozone reacts with bromide to form brominated organic compounds, as well as bromate. Also, lower valent transition metals such as Fe2+ and Mn2+ will consume ozone.
Oemke and van Leeuwen (in “Chemical and Physical Characteristics of Ballast Water: Implications for Treatment Processes and Sampling Methods,” CRC Reef Research Centre, Technical Report No. 23, 1998, and in “Potential of Ozone for Ballast Water Treatment,” CRC Reef Research Centre, March 1998) have identified several potential problems when ozonating marine ballast waters, the most serious of which is reaction with bromide. The bromide concentration in seawater is 1.915 mg/L per % salinity. They estimated a significantly high bromide concentration of 40 mg/L in ballast water sampled during their work. The reaction chemistry of ozone with bromide is complex, and involves a cyclic decomposition reaction with bromide that simply consumes ozone and regenerates bromide. The reaction is mediated by the hypobromite ion. This anion undergoes an additional reaction with ozone to form bromate. In this reaction, one mole of hypobromite consumes two moles of ozone. Hypobromous acid is a weak acid with a pKa of 8.8 at 20° C. The conjugate acid of hypobromite does not further react with ozone, so the authors suggest that lowering the pH to about 7 would assist in quenching the cyclic decomposition and the formation of bromate. The bromide reaction is further complicated in the presence of dissolved organic matter. The reaction paths become so complex that empirical models are required to predict the amount of bromate that forms. Organobromine compounds form, as well, a concern from a health and environmental standpoint.
The concept of shipboard treatment of ballast water presents a number of challenges that any technological solution would typically need to address. These challenges include a variety of constraints and performance expectations. First, large volumes (up to 10 million gallons), which are routinely taken on and discharged as part of the normal ballast operation of the ship, must be treated at a rate high enough to keep up with the ship schedule. Second, both macroscopic organisms and microorganisms and their spore and cyst forms must be killed or inactivated. Third, the quality of ballast water typically is poor, with high turbidity and a difficult chemical matrix. Fourth, both space and power are limited aboard a typical ship.